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Heavy media separation (HMS) is one of the most widely used gravity methods in the mineral industry. The feed is separated, according to the specific gravities, into two products; float (light) and sink (heavy) particles. The universal acceptance of the process is due to simplicity, efficiency and robustness for ease of control. However, if optimum separation is desired during the operation of heavy media plants, a delicate balance of a number of variables is essential. Among the variables, the two which are generally the most frequently checked and controlled are specific gravity and consistency of the medium.
Rheology of Heavy Media
The finely ground solids dispersed in water make up the heavy media suspension. The particles must be such a size, generally not greater than 65 mesh, so that they can be readily maintained in homogeneous distribution throughout the fluid. Heavy media suspensions of this type, volumetric concentrations greater than 25%, do not follow the same rules of fluid mechanics as simple fluids (like water or oil), therefore to understand their behavior viscosity will be defined.
When a body is moving in a fluid, it experiences a viscous drag or retarding force. The amount of viscous drag is a function of the speed by which the body moves. Mathematically:
T = n dv/dy
where T is the applied force/unit area or shear stress required to move the body (dynes/cm²)
dv/dy is the change of velocity with distance of the moving body or shear rate (sec-¹)
n is the coefficient of viscosity (Poise or centipoise)
T = VB + npl dv/dy
where VB is the yield stress (dynes/cm²) npl is the plastic viscosity.
Characteristics of Biopolymers
Recently biopolymers have been developed and introduced in the oil well drilling industry with great success. They generally exhibit a low viscosity at the bit, yet provide adequate viscosity in the annular space to ensure good hole-cleaning and cuttings-carrying ability. This pseudoplastic (shear thinning) nature of a 1% solution can be seen from the drop in consistency reading (apparent viscosity) with increased speed rate of shear of the viscometer. It is this pseudoplastic characteristic that contributes so greatly to the suspending properties of polymers. The solution structure is sufficient to prevent fine particles from settling, yet these solutions pour readily, and in many ways appear to be much thinner than solutions of other gums which provide poorer suspensions.
Operation of HMS plants is under better and easier control; easy startup, good pumping characteristics. As only a very small amount of polymer is required compared with other agents to stabilize media, costs would be very low. As polymers dissolve in water, and practically all the water is retained in the separator, no appreciable polymer loss would be incurred.
Results and Discussion of Consistency Experiments
The experiments were set up with the aim of studying the rheological behavior of heavy media suspensions using the equipment described in earlier sections to obtain data which would be useful in practice to plant operators for better control of their plants. Kelzan XC was chosen as a polymer as it was readily available, although there are several similar materials on the market. The heavy media constituents used were magnetite, magnetite-ferro-silicon in 50/50 ratio and ferrosilicon alone. Stability and consistency parameters were measured in the three types of media with different specific gravity and several levels of polymer concentrations.
Before discussing results, the significance of stability has to be clarified. In the previous section, the stability measuring apparatus was described and the two limiting parameters – 0 to 100% stability – were defined. In practice, neither zero nor 100% figures are met and generally the working range of an HMS vessel is between 50-80%. At 70% a density differential of 0.10 s.g. unit between float and sink fractions is an acceptable figure, and a very good separation is achieved.
All three types of media; magnetite, magnetite/ferrosilicon (50/50) and ferrosilicon, considerably improved their stabilities with very small amounts (less than 0.1%) of polymer addition. There seemed to be a maximum attainable stability for a given medium specific gravity beyond which flocculation took place and this was characterized by a faster settling rate. The essence of the result is that there is no practical advantage to be gained by adding polymers in larger quantities than 0.1% except for densities of medium less than 2 gm/cm³.
Plastic viscosity is the other important rheological parameter of the heavy media suspension, not from the stability but from the separation efficiency point of view. When a particle is held in a suspension, the yield stress is responsible for it, but when it is moving in any direction, its velocity is a function of plastic viscosity. This means that plastic viscosity must be kept as low as possible for the feed to move as fast as practically required for efficient and accurate separation.